「for」這隻字幾時會放在 問句的最後......

前面加個what，即係「what for?」
意思係「這究竟為了什麼?」

What for? spoken English

What is it for ? = What is the usage of it? Proper written English

What do you look for ? simple present tense

Whom do you wait for?

What do you wait for?

What are you looking for? present continuous tense

Whom are you waiting for?

What are you waiting for?

其實，問句是由普通句子變成的。
我們只把原句的字逐隻搬到正確位置，再加適當的question word，最後加上問號即可。(加問號是很重要的，別輕看)
I am looking for x.
What are you looking for?
基本結構為：
question word>auxiliary verb>subject>verb及剩下的>問號
What are you looking for ?

這是現代口語常用的結構，還有一種結構，常用於正規文件，有些守舊的文代家認為preposition不能寫在句子的最後，一定要用這結構：
preposition>question word>auxiliary verb>subject>verb及剩下的>問號
For what are you looking ?

有一種例外情況，where why when，它們不跟preposition。
You are going to x.
Where are you going?
用what: To what place are you going?/ What place are you going to?

I did it for x.
Why did you do it?
用what: For what reason did you do it?/ What reason did you do it for?

請參考以上，when可用at what time取代(當然不同情況用不同的preposition)。

還有樓上所說的What...for?
What...for?用來問原因，與why相似，例如
"We need a bigger car." "What for?"
What are you doing that for?

希望以上對你有幫忙^^

2010-08-22 16:32:29 補充：
可能我說得不太清楚，你問什麼時候會需要將"for"放在最尾，

其實，問句是由普通句子變成的。
我們只把原句的字逐隻搬到正確位置，再加適當的question word，最後加上問號即可。(加問號是很重要的，別輕看)
＊所以原句有for在最後，我們就把for放在最後。＊